1. Field of the Invention
The present invention relates to a print head that ejects a liquid onto a print medium.
2. Description of the Related Art
In recent years, ink jet printing apparatuses have been prevailing rapidly in which a print head is located opposite a print medium to eject ink droplets onto the print medium for printing. The ink jet printing apparatuses have the advantages of being easily miniaturized and being able to relatively easily perform color printing. An ink jet printing apparatus is disclosed in, for example, Japanese Patent Laid-Open No. H04-10941 (1992). Such an ink jet printing apparatus uses any of various techniques for improving the quality of images resulting from printing.
When the ink jet printing apparatus is used for printing, a dot density control method can be used for printing; the dot density control method controls the number of print dots of a given size provided per unit area in order to express an intermediate gray level. For such control means, a printing method has been proposed which uses nozzles for ink droplets of different sizes so that smaller ink droplets are used to form print dots for the range from a bright portion to an intermediate gray level portion of the image, whereas larger ink droplets are used to form print dots for the range from the intermediate gray level portion to a dark portion of the image.
For example, Japanese Patent Laid-Open No. 2003-311964 discloses an ink jet printing apparatus using, for printing, a print head in which ejection ports involving ejected droplets of different flow rates are formed, as described above. The print head in such an ink jet printing apparatus is known to have plural types of ejection ports formed therein under different conditions; the diameter of the ejection port, the sectional area of an ink channel, and flow resistance in the print head vary among the ejection ports. This enables plural types of droplets to be ejected.
On the other hand, there has been a demand for improvement of the quality of images provided by the ink jet printing apparatus. Efforts have been made to reduce the size of droplets ejected during printing. However, when droplets of a reduced size are ejected, as is, a reduced amount of droplets are ejected, preventing supplying of an amount of droplets required per unit area sufficiently. Thus, when the diameter of the ejection ports is reduced to decrease the size of the droplets, nozzle row resolution correspondingly needs to be increased. However, the increase in nozzle row resolution is limited. In general, a further reduction in the size of the droplets ejected is known to reduce ejection efficiency with respect to the droplet ejected. Furthermore, as nozzle density is increased to improve the nozzle resolution, the size of heaters more significantly affects the nozzle resolution in connection with the array of the nozzles. With a further increase in nozzle density, the array of the nozzles is affected by the size of the heaters per se. Then, that makes it difficult to connect between wiring and the heaters. Finally, the heaters cannot be arranged in a line. This applies not only to the heaters but also to the channels though which ink is fed. It is regarded as the size of the ink channel per se prevents an increase in nozzle density.
Thus, an invention has been disclosed in which heaters are alternately staggered and a plurality of ejection ports with different dot diameters are also staggeredly arranged as disclosed in Japanese Patent Laid-Open No. 2005-1379.
An invention has also been disclosed in which heaters are alternately staggered with the flow resistance of flow channels improved as disclosed in Japanese Patent Laid-Open No. 2006-315395.
However, in spite of these measures, the quality of images resulting from printing may be degraded.
For example, in print heads in some ink jet printing apparatuses, ink supplied to an ink supply port is fed to a common liquid chamber. The ink is then fed, via ink channels, to pressure chambers inside which respective print elements are arranged. In this case, the print head adopted may be of a type in which the channel width in the pressure chamber is larger than that in the ink channel. In this manner, the ink channels, each extending between the corresponding pressure chamber and common liquid chamber in the print head, have a relatively small channel width. This enables an increase in the flow resistance of ink stored inside the ink channel, preventing movement of the ink in the ink channel. Since the ink in the ink channel has difficulty moving, when an energy generating element is driven inside the pressure chamber, the resulting bubbling energy is inhibited from escaping toward the common liquid chamber. As a result, most of the energy generated by the energy generating element is used to eject the ink. The ink is thus efficiently ejected.
However, when the ink channel has a smaller channel width than the pressure chamber, a wall surface is formed at the boundary between the pressure chamber and the ink channel toward the interior of the channel. Then, a bubble generated during driving of the energy generating element may come into contact with the ink supply port side of the wall surface of the pressure chamber. Upon coming into contact with this side of the wall surface, the bubble may be deformed and become asymmetric. Thus, the shape of the bubble may become disproportionate, preventing the ink from being ejected straight.
If the ink fails to be ejected straight as described above, a trailing portion of the ink may be partly torn away and separated from the main droplet to form a sub-droplet (satellite droplet). In general, in the ink jet printing apparatus, when the print head ejects the ink, the ejected droplet is divided into the main droplet and the trailing sub-droplet called the satellite droplet. The satellite droplet thus generated flies in a direction different from that of the main droplet. As is known, the satellite droplet impacting a print medium may affect the granular property of a printed image. Dot density may then be varied to cause an uneven density, stripes, or the like at a scan boundary. The printed image is thus affected.
At present, to prevent the impact position deviation from affecting the printed image, the speed of a carriage is reduced to substantially mitigate the adverse effect of the satellite droplet. Furthermore, the number of passes of the multipass printing during scanning is reduced to decrease print speed. However, to increase the print speed with the high quality of the printed image maintained, the impact accuracy not only of the main droplet but also of the satellite droplet needs to be improved.
Also disadvantageously, if the size of satellite droplets continues to decrease in connection with the recent decrease in the size of ejected droplets, the satellite droplets fly in and around the printing apparatus and may adhere to the apparatus. The satellite droplets adhering to the printing apparatus disadvantageously contaminate the interior of the apparatus.